update API doc with +API (h1 heading)

doc/api/access_requests.md

-# Group and project access requests
+# Group and project access requests API
  >**Note:** This feature was introduced in GitLab 8.11

doc/api/enviroments.md

-# Environments
+# Environments API
 ## List environments

doc/api/groups.md

-# Groups
+# Groups API
 ## List groups

doc/api/issues.md

-# Issues
+# Issues API
 Every API call to issues must be authenticated.

doc/api/keys.md

-# Keys
+# Keys API
 ## Get SSH key with user by ID of an SSH key

doc/api/labels.md

-# Labels
+# Labels API
 ## List labels

doc/api/members.md

-# Group and project members
+# Group and project members API
 **Valid access levels**

doc/api/merge_requests.md

-# Merge requests
+# Merge requests API
 ## List merge requests

doc/api/notification_settings.md

-# Notification settings
+# Notification settings API
 >**Note:** This feature was [introduced][ce-5632] in GitLab 8.12.

doc/api/projects.md

-# Projects
+# Projects API
 ### Project visibility level

doc/api/settings.md

-# Application settings
+# Application settings API
 These API calls allow you to read and modify GitLab instance application
 settings as appear in `/admin/application_settings`. You have to be an

doc/api/snippets.md

-# Snippets
+# Snippets API
 > [Introduced][ce-6373] in GitLab 8.15.

doc/api/todos.md

-# Todos
+# Todos API
 > [Introduced][ce-3188] in GitLab 8.10.

doc/api/users.md

-# Users
+# Users API
 ## List users